Anyone who has ever played football, or drunk, with Roy Keane has a favourite story, and Lee Sharpe's goes back to the days when his Manchester United team-mate regularly used to punch and puke his way through the city's nightspots. Keane had been knocking back the drink one Saturday night, without any food, and was "absolutely smashed", according to Sharpe, when some Liverpool players walked into the bar.

It was Liverpool's Spice Boys white-suits-for-the-cup-final heyday and Keane started picking an argument with his fellow Republic of Ireland international Phil Babb. Sharpe can remember him shouting (with the expletives removed): "Who the hell do you think you are, Babb?"

John Scales tried to act as peacemaker and walked into another stream of invective. "You, Scales, you're rubbish as well, with your England B cap, you're nowt, rubbish." Then Jamie Redknapp came over, with his London patter and twinkling eyes, trying to smooth things over. "You, Redknapp, are you happy with your Under-21 caps?" Keane spluttered. "What the hell have you done in the game?" On and on it went. "All of which I thought was pretty funny," Sharpe cheerfully remembers. "Roy was a great lad, a great mate. He loved a laugh and to go out, but he had this blackness inside him I never understood, and it came out when he drank."

The story does not reflect well on Keane but it is a measure of his status that few United supporters will cringe at the memories of his worst excesses. Many, in fact, will just chuckle knowingly. This is the thing about Keane: even when he acted like a fool he somehow emerged with his immortality enhanced. He may have black edges to his heart but the fact is that a large part of his following at Old Trafford always admired his hardness and, when it came down to it, his willingness to express himself in violence.

"United fans take the rogue over the role model every time," says Richard Kurt, the author of Red Devils, a history of the club's most notorious players. "We have cool respect and admiration for Bobby Charlton but it is Keane and Cantona and Best whom we love." And it will be Keane's name that will be bellowed the loudest at Old Trafford today, when he takes his place in the away dugout to face Sir Alex Ferguson for the first time as a rival manager.

"Even now," says Kurt, "one of the most popular T-shirts for sale outside the ground is of Roy. The picture is classic Keane: in the tunnel at Highbury, leaning over the referee's shoulder, pointing at Patrick Vieira and screaming: 'I'll see you out there!'"

It is almost two years since Ferguson expelled Keane, to the horror of most supporters, because of what he deemed an act of treacherous disloyalty, namely an interview on MUTV that was so critical of the team it had to be pulled from the schedules. The two men have since made up and share a businesslike relationship.

But the intriguing fact is that Keane's relationship with the other players was never badly damaged by the episode. Just as the fans have remained behind their former captain, nobody from the dressing room has ever said anything remotely negative about Keane, and several players, including Wayne Rooney, visited him in Glasgow when he was playing for Celtic. Tellingly, Rio Ferdinand, one of the players castigated on MUTV, has nothing but praise for him in his autobiography. "I reckon he was the most influential player there has ever been in the Premier League," he wrote.

Steve Bruce, who played alongside Keane for five years, agrees. "Roy was the complete midfield player of his time," says the former centre-half. "He could score, defend, tackle, run, pass. He had the lot, but he also had an unbelievable desire to win and that's what made him stand out more than anything. He will relish going back and he will get a great reception."

"Respect" is the key word when United players, past and present, talk about the Keane factor. "Respect, but also fear because of the player he was and the aura he had," says Danny Higginbotham, who began his career at Old Trafford and has just signed for Keane's Sunderland for £2.5m from Stoke City. "You weren't terrified of him; I just mean it was a massive amount of respect. He would lead by example and if he had a go at you you would take it because you respected the way he carried himself. You could do nothing but learn from him. And he wouldn't just have a go at people for the sake of it. Yes, he would have a dig if he needed to but he would also tell you when you'd done well."

Ferguson, who has spoken of Keane being "almost like family", has been asked more than once over the past year whether he could imagine the Irishman replacing him. It is not a question he is willing to answer, but one suspects he would not be against the idea. "It was not an easy parting of the ways for either of us," he says, "but it takes nothing away from a fantastic association I enjoyed as his manager for 12 years. He was one of the club's great captains and I did not describe him as the heartbeat of Manchester United for nothing."